This invention relates to an improved process for the purification of crude isopropyl alcohol.
Isopropyl alcohol (IPA) is well known to be conventionally manufactured by processes which involve the hydration of propylene. Hydration may be direct or indirect. In direct hydration, propylene is reacted catalytically with water. Suitable catalysts for this purpose are known to include cation exchange resins, concentrated phosphoric acid, silicotungstate compounds, etc.
The indirect hydration process, which is of particular interest in the application of the present invention, comprises steps for the reaction of propylene with sulfuric acid to produce propane sulfate followed by the hydrolysis of the sulfate to the desired IPA.
The crude product which results from propylene hydration processes typically contains not only the desired IPA but also diisopropyl ether, polymers (e.g., propylene trimers, tetramers, pentamers, etc.), and water. Organic sulfur derivatives are also found in the product of the indirect hydration process. In a typical operation the crude product might contain 60 percent by weight (% w) IPA, 6% w diisopropyl ether, up to 0.5% w of the polymers and sulfur derivatives, small amounts of acetone and propane, and the remainder water.
In conventional practice, the crude IPA from indirect propylene hydration to IPA is commonly treated for purification, or finishing, by distillation and/or extractive distillation to remove the diisopropyl ether and other impurities.
The principal object of this invention is improvement of crude IPA finishing. In one respect, it is particular object of the invention to reduce the difficulty and expense associated with operations applied in the prior art for the finishing of crude IPA. Conventional distillation practices have proven to be very costly both in terms of the necessary equipment and the energy required to operate it, e.g., energy necessary to vaporize not only the organic ether and IPA products but also the large content of water accompanying the IPA during finishing. For example, one typical prior art practice for purification of crude IPA involves feeding the crude stream from the propylene hydration to a multi-tray extractive distillation column, using a water extractant, to distill overhead most of the diisopropyl ether and other impurities, leaving a bottoms product containing about 15% IPA in water. This IPA solution is then subjected to multi-stage distillation to recover a distillation overhead consisting essentially of an azeotropic mixture of IPA and water. The IPA/water solution is then subjected to azeotropic distillation with an added azeotroping agent (for instance, finished isopropyl ether) for dehydration. Downstream of azeotropic distillation, remaining heavy components, including C.sub.6 to C.sub.12 polymers and diisopropyl ether, are rejected in a further multi-stage distillation.
In another respect, it is a particular object of the invention to improve the effectiveness of the removal of certain impurities from the crude IPA. In this respect, it is of particular interest to enhance the performance of the finishing process for the removal of organic sulfur derivatives from the product of the indirect hydration process. Even when present in only very small quantities, sulfur derivatives often impart a very undesirable odor to the finished IPA. Difficulties associated with sulfur removal may result in products which do not satisfy stringent specifications required for IPA in many end uses, e.g., its use in the formulation of cosmetic and pharmaceutical products.